1. Field of the Invention
The present invention relates to a microlens array used for a screen of an optical apparatus or focusing screen of a single lens reflex camera.
2. Description of the Related Art
Conventionally, it has been known that a microlens array having a plurality of microlenses arranged therein is used as a screen or focusing screen. A microlens array screen has an advantage that it has less granularity than a screen with a microscopic irregularity transferred from an abrasive-machined surface of a mold, and therefore, can provide a clear view.
However, if the microlenses are arranged regularly, a problem arises in that diffracted light is limited to a particular direction and thus an unnatural out-of-focus is provided, or when the microlens array is used with a Fresnel lens, it interferes with an orbicular zone of the Fresnel lens to produce moirxc3xa9 fringes.
As for the microlens array used for the above-described purpose, this problem can be solved if the microlenses or micro concave parts are arranged at random.
In Japanese Patent Laid-Open No. 11-142609 and the like, there are proposed methods of providing a microlens array with a random arrangement.
However, as described later with reference to a method of manufacturing a microlens array, these methods have problems associated with stability in manufacture, efficiency and controllability and are considered difficult to implement.
The conventionally known methods of manufacturing a microlens array include an ion exchange method of forming a plurality of lenses by increasing refractive index of a substrate of a multicomponent glass at a plurality of areas distributed thereon (M. Oikawa, et al., Jpn. J. Appl. Phys. 20(4) L51-54, 1918), and a method of subjecting a photosensitive glass to a heat treatment to crystallize non-sensitized parts thereof, thereby swelling the surface.
With these methods, however, an aperture of each lens cannot be made large with respect to an interval between the lenses, and a radius of curvature of each lens is also small. Thus, it is difficult to design a lens with a large numerical aperture.
Besides, manufacture of a microlens array with a large area needs a large scale manufacturing apparatus. Thus, it cannot be readily manufactured.
As another method, there is a method of manufacturing a microlens array by fabricating an original plate of the microlens array, spreading a material of the lens onto the original plate, and then peeling the material of the lens. To fabricate a mold serving as the original plate, a process of lithography using an electron beam (see Japanese Patent Laid-Open No. 1-231601), a process of shaping a part of a metal plate by etching (see Japanese Patent Laid-Open No. 5-303009), and a process of shaping a metal plate by impression using an indenter (see Japanese Patent Laid-Open No. 11-142609) may be used.
According to this method of manufacturing a microlens array, the microlens can be duplicated by molding, variation from lot to lot hardly results, and the microlens array can be manufactured at low cost. In addition, compared to the ion exchange method, problems associated with a coefficient of thermal expansion, warpage and the like can be avoided.
However, in the case of the process involving an electron beam, it is difficult to fabricate an original plate with a large area of 100 cm square or larger, because an electron beam lithography device is expensive so that a significant capital investment is needed, and an area for lithography is limited.
In the case of the process involving etching, isotropic etching using a chemical reaction is typically used, and thus, even a little change in a composition or crystal structure of the metal plate results in a failure in the etching to provide a desired shape.
In addition, in the case of the process involving etching, the etching continues if washing is not conducted immediately after the desired shape is provided. Thus, when forming a microlens, the resulting shape may be different from the desired one due to the etching advanced from the point in time when the desired shape is provided until the washing.
In the case of the process of impression using an indenter, the indenter is pressed against the mold a number of times corresponding to the number of lenses, and thus, an enormous amount of time is needed to provide a large area. In addition, the microscopic precision thereof is limited mechanically, and the controllability on the order of micrometers is hardly attained.
With many of these conventional methods of manufacturing a microlens, it is extremely difficult to arrange the microlenses randomly in a controlled manner, and thus, the microlens arrangement is poor in reproducibility.
Even if the microlenses are arranged regularly, the conventional methods of manufacturing a microlens have many unreliable factors including the reproducibility of a lens radius, which is an essential parameter of the microlens, and the precision of the position of the arranged lens. Thus, quality control is quite difficult in mass production.
An object of the present invention is to provide a microlens array with bright and clean viewing characteristics and a high reproducibility of a shape of each microlens.
In order to attain the above object, in the microlens array of this invention, a plurality of microlenses are formed with an irregularity or probability-distributed regularity so as to differ from a hypothetical base pattern, the hypothetical base pattern having apexes of hypothetical microlenses each spaced apart from every adjacent microlens thereof by an equal distance L. In addition, all the apex positions of the microlenses are arranged to lie within circles each centered on an apex position in the hypothetical base pattern and having a radius equal to or smaller than 0.3L  less than  . . . (1) greater than , or meet a condition:
0.4Lxe2x89xa6Pxe2x89xa61.6Lxe2x80x83xe2x80x83(2).
Thus, a microlens array with bright and clean viewing characteristics and a high reproducibility of a shape of each microlens is provided, which does not suffer from an influence of the diffracted light and occurrence of the moirxc3xa9 strings specific to the cyclic regular arrangement, such as the base pattern, as well as, if the above conditions (1) and (2) are met, local darkening due to a too long distance between the apexes of the adjacent microlenses and granularity caused by the darkening, and local poor diffusion due to a too short distance therebetween.
A detailed configuration of the microlens array, optical apparatus, single lens reflex camera and optical finder of the invention, the above and other objects and features of the invention will be apparent from the embodiments, described below.